charctestic of H-Bonding
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  Characteristics of Hydrogen bonding  1) The hydrogen atom participating in H-bond must be covalently bonded to a highly electronegative atom in order to acquire significant positive charge. For example, H atom covalently bonded to electronegative atoms like F, O and N can participate in hydrogen bonding. However, H atoms bonded to less electronegative atoms like B, C etc. or larger electronegative atoms like S, P, Cl, Br etc. cannot get sufficient positive charge and hence cannot enter into H-bonding. Nevertheless there are some special cases. 2) The acceptor electronegative atom should also possess at least one lone pair and high charge density so as to attract positively charged H atom effectively. Thus H-bonding is normally observed with smaller and highly electronegative atoms such as F, O and N with lone pairs. 3) The H-bond is mostly electrostatic in nature. However, directional nature of hydrogen bonding, shorter inter atomic distances between molecules participating in H-bonding and limited number of H-bond interactions suggest some covalent nature also. E.g. In HF, the number of H-bonds per each molecule is limited to only two, since there is only one hydrogen per molecule and only one lone pair on Fluorine can participate in H-bonding. The directional nature is also evident from the following diagram. 4) The hydrogen bond is stronger than van der Waal's forces of attraction. But it is weaker than the covalent bond and ionic bond.  The strength of Hydrogen bonds can range from very weak (1 – 2 kJ mol -1 ) to extremely strong (161.5 kJ mol -1 ). H-bonds involving fluorine are associated with higher energies. Most of the remaining types are below the range of 21-25 kJ mol -1 . 5) The strength of H-bond is directly proportional to the electronegativity and the ability of electronegative acceptor atom to contribute the electron density. HYDROGEN BONDING Definition:  The electrostatic attraction between partially positively charged hydrogen atom bonded to a relatively electronegative atom and another electronegative atom is referred to as hydrogen bond  .  It is represented by a dotted line as shown below.   * Z is an electronegative atom to which H atom is covalently bonded  . * The H-Z group is also called as donor group  since it provides hydrogen atom for H-bonding. * X is another electronegative atom that attracts positively charged H atom. * X is referred to as hydrogen bond acceptor   since it accepts positive charge from the H atom. * The acceptor atom, X may or may not be bonded to another H atom covalently but should contain at least one lone pair on it.   Note:  The donor and acceptor terminology can be a bit confusing since we are used to think about chemical bonds in terms of electrons. The H-bond donor provides H atom for making hydrogen bond but accepts the electron lone pair, whereas the H-bond acceptor receives H-atom and is actually the electron lone-pair donor. * X and Z are smaller and highly electronegative atoms. They are usually, but not necessarily, from the first row of periodic table i.e. F, O and N. The δ+ H atom attracts the lone pair el  ectron density on X. This predominantly electrostatic force of attraction between δ+ H and Z is called   as hydrogen bonding and is represented by a dotted line.  
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